Angular deviations in products formed by bending with a press are caused by sagging of the bending means whereby the bending tool can penetrate less deeply in places into the lower tool. The result is that the bending angle is not constant over the length of the product formed by bending.
Conventional methods by which the sagging is compensated comprise forming a curve on the upper and/or lower tool, whereby a certain sheer is given thereto. An example of such curve-forming means consists of a system of pairs of co-acting wedges which are arranged over the lengthwise direction of the bending means and which can cause a height adjustment in transverse direction thereof.
Another example of a known curve-forming means comprises a curve-forming strip with a great number of wedge surfaces arranged in lengthwise direction of the bending means, which strip is slidable in lengthwise direction and a height adjustment is possible with the separate wedges located on the wedge surfaces.
It has been found however that compensating for the predictable sagging with the known curve-forming means does not always lead to an optimal compensation. The bending means in any case comprise, in addition to the upper tool and the lower tool, a number of auxiliary elements such as an upper beam, a spacer, a table, a lower beam, and the like, which naturally display irregularities that are not predictable and are susceptible to change in the course of time. In other words, as a result of differences in tolerances of the mechanical elements used, it is in practice not possible to fully compensate with a curve-forming device according to the state of the art such that the product formed by bending possesses a substantially constant bending angle over its length.
It is noted that the combined use of the above mentioned curve-forming means acting in transverse direction and in lengthwise direction would lead to the desired result, but involves a great number of undesired practical drawbacks (long and repetitious adjustment process).